The Smith-Lemli-Opitz syndrome is a developmental disorder that affects many parts of the body. This alteration is characterized by microcephaly, learning disabilities or mental disabilities, and behavioral problems. Many affected children have the characteristics of autism. There are also frequent malformations of the heart, lungs, kidneys, gastrointestinal tract and genitals. Infants with this syndrome have hypotonia, feeding difficulties and tend to grow more slowly than normal. Most affected individuals have syndactyly, and some polydactyly.

Signs and symptoms of the syndrome vary widely among affected others. Some of those affected may have minor physical anomalies with learning and behavior, while in the most severe cases, can be life threatening and involve a profound intellectual and major physical abnormalities disability.

This process is due to mutations in the gene DHCR7, located on the long arm of chromosome 11 (11q13.4). This gene encodes an enzyme called 7-dehydrocholesterol reductase, which is responsible for the final stage in the production of cholesterol. Mutations in the gene DHCR7 reduce or eliminate the activity of the 7-dehydrocholesterol reductase, preventing the cells produce enough cholesterol. The lack or absence of this enzyme causes potentially toxic by-products accumulate cholesterol in the blood, nervous system, and other tissues. The combination of low concentrations of cholesterol and other substances build up may stop the growth and development of many body systems.

They have identified more than 120 mutations in the gene that cause DHCR7 Smith-Lemli-Opitz syndrome. The most common mutation, IVS8-1G> C, alters a single nucleotide in the gene. This change interferes with the normal process of the 7-dehydrocholesterol reductase. Often, another mutation replacing the threonine amino acid, methionine, at position 93 of the enzyme (Thr93Met or T93M) occurs.

This disease is inherited in an autosomal recessive pattern, that is, both copies of the gene in every cell must have mutations for alteration is expressed. The parents of an individual with an autosomal recessive disease have a copy of the mutated gene, but usually show no signs and symptoms of the disease.